It is desirable that the operating lives of electrolytic capacitors be as long as possible. The use of high voltage electrolytic capacitors particularly has increased considerably in recent years. These capacitors are used in large numbers in many devices such as power supply devices, inverters and charge-discharge devices. Increasingly, such devices require longer operating lives, low failure rates, lower ESR (equivalent series resistance) values, as well as high energy densities.
It is particularly important that during their operating lives the rate of failure of such capacitors be held to a minimum. The reason for this is that many times high voltage capacitors are used in a bank in which they are connected in parallel. Thus if one of the capacitors should short while the bank is at operating voltage the other capacitors discharge into it. As a result, the shorting capacitor is frequently totally destroyed. In such a case the device in which the capacitors are employed becomes inoperative.
The failure of such a device may have serious consequences, for example when the device is a power supply and is used in a computer. Here failure of the power supply may result in a loss of memory.
Besides the above-mentioned requirements it is an additional requirement that the electrolyte be free of toxic substances.
Capacitors provided with such an electrolyte are shown in Dapo, U.S. Pat. No. 5,111,365. The electrolyte shown in this patent contains N-methylformamide, 2-methoxyethanol, 2-ethoxyethanol, ethylene glycol or 1,2-propylene glycol, an aromatic dicarboxylic acid such as isophthalic acid or terephthalic acid and an aliphatic amine such as dimethylamine or monomethylamine. These capacitors have been found to be useful at relatively low voltage applications, for example about 55VDC.
Finkelstein, U.S. Pat. No. 4,373176 shows electrolytic capacitors for use at operating voltages of 200VDC. In these capacitors the electrolyte employed contains a tertiary amine or a dipropylamine mono salt of dodecanedioic acid dissolved in a solvent consisting of ethylene glycol, N-methyl-2 pyrollidone and water. While these capacitors are free of toxic substances, their operating lives are too short or their ESR values are too high for many applications.